1. Field of the Invention
The present invention relates to the field of ferrous and nonferrous metal casting and in particular to the debonding and removal of sand cores from cast parts, and in some cases, the heat treating of the cast parts in conjunction with the removal of sand cores.
2. Brief Description of Related Art
In the casting of ferrous and nonferrous metals into parts, the foundries in the United States consumed 7.7 million tons of foundry sand in the year 1988 alone.
The steel foundries and many of the gray iron foundries use high purity (over 98% by wt. SiO.sub.2) silica sand for casting molds. Many of the automotive foundries use a less pure (over 93% by wt. SiO.sub.2) silica sand.
Most of this sand is used by the foundries for molding or core making. When making molds or cores, a binder material is added to the foundry sand to form the mold or the core. In general, the mold forms the outside surfaces of the casting, while the cores form the inside surfaces and paths.
The cast part is formed by pouring the molten ferrous or nonferrous metal into the mold. When the part has internal openings or paths, the molten metal is poured into the volume between the mold and the core(s) usually surrounding some or most of the core. When the metal solidifies, the mold is opened and the part is removed. In most cases, the core remains in the interior regions its presence has formed and must be removed.
Removal of the cores is usually accomplished by impact and vibration devices, and/or by heating to destroy the binders and/or manually by breaking and prying out of the cores. The cores are generally broken into smaller pieces within the part and can be removed through various part openings. The degree of difficulty of doing this "sand core debonding" depends upon the geometry of the part being cast.
There are a number of binder systems used for the bonding of foundry sand into sand molds and sand cores, the major ones being identified in the trades by the following nomenclature:
Furna No-Bake PA1 Phenolic Urethane Cold Box PA1 Penolic Urethane No-Bake PA1 Shell Resin PA1 Phenolic Hot Box PA1 Silicates (No-Bake & CO.sub.2) PA1 Phenolic No-Bake PA1 Alkyd Oil No-Bake PA1 Core Oil
The "No-Bake" sands are ones in which foundry sand, plus a binder and an appropriate catalyst or accelerator produces a moldable sand mixture which hardens by a chemical process. The other type binder systems are heat cured for hardening. Cold box binder systems cure very rapidly in seconds due to chemical reaction between components of the system. These systems are well known and used by those skilled in the art.
In most cases the mold is used once and is destroyed by the casting process. In all cases the sand core is destroyed by the casting process or by the method used for the sand core debonding from the cast part.
As those skilled in the art will appreciate, the debonding and removal of sand cores from a complex cast part is a difficult and costly process relative to the other aspects of a ferrous or nonferrous metal casting operation.
In the case of casting parts of aluminum or aluminum alloys, it is particularly difficult to remove the sand core because of the lower casting temperature used. A lower interface temperature, usually results in less separation of the sand core from the aluminum part. The aluminum also is a softer material and more prone to damage if physical impact is used in the debonding and removal process. In addition, it is necessary to cool the aluminum part substantially before any attempt is made to debond and remove the sand core by any reasonable physical means, or the soft part will be damaged by even modest handling.
When heating methods are used to remove sand cores by thermal destruction of the binder systems, heating cycles are typically long, 4 to 10 hours, and the removal of the core is frequently incomplete. Pieces of sand core remain where the heating process did not effectively thermally decompose all parts of the sand core.
Additionally, sand core material removed from the castings must be disposed of or reclaimed. Disposal has become increasingly expensive because the binder residue is usually classified as a hazardous and/or toxic waste which must be handled accordingly. Reclamation of the foundry sand through physical and thermal processing steps is receiving increasing attention, but also involves a significant cost.
This invention involves the use of the fluid bed furnace for the removal of sand cores used to form internal passages and other details when casting metal parts. This invention eliminates the major processing disadvantages encountered with existing methods employed for sand core debonding by:
1. requiring no hammering or impacting of the parts to break-up or loosen the sand cores; PA0 2. debonding the sand cores in a way that eliminates toxic and/or hazardous components introduced with the binder system thereby eliminating an environmentally adverse impact of the foundry industry; PA0 3. in the majority of the cases, recovers the sand used to form the sand core sufficiently pure so that it is unnecessary to dispose of, reclaim separately, or replace this material in foundry processing; and PA0 4. by making it technically feasible to introduce the cast parts into the sand core debonding process immediately after casting and while the parts are still at a sufficiently high temperature to require little or no energy input of the sand core debonding process.
Fluid bed furnaces are well-known in the metal treatment arts for their advantages of rapid and uniform heat transfer, ease of use, and safety. See, for example, U.S. Pat. No. 3,053,704 which is incorporated herein by reference thereto. Conventional fluidized bed furnaces may comprise a retort or treating vessel containing a finely divided particulate solid heat transfer medium, e.g. silica sand or aluminum oxide. A distributor plate is positioned at the lower end of the retort for introducing fluidizing gas to the retort upwardly through the bed media from a plenum chamber below. The fluidizing gas suspends the bed media in an expanded mass that behaves like a liquid. Heat is transmitted to the expanded mass from electric heaters, or the like, either directly or through the walls of the retort and/or the fluidizing gas may be heated before it enters the retort. A workpiece submerged in the heated expanded mass is rapidly and uniformly heated.
The workpiece(s) are typically placed in fixtures or baskets which are submerged in the fluidized bed furnace fluidized medium.
In the fluidized bed furnace, the parts can be heated or cooled very uniformly through conduction of heat into the fluidized medium and/or by adjusting the temperature of the fluidizing gas causing fluidization of the particulate medium.
In addition, the surface of the parts and the particulate medium are subjected to the sweeping action of the fluidizing gas.
This invention uses the properties of the fluidized bed furnace system to advantage in the debonding and removal of sand cores from cast parts, and in some cases, for the heat treating of the cast parts in conjunction with the removal of sand cores.